news and v iews

512 volume 41 | number 5 | may 2009 | nature genetics

discovered genotypes will get harder before it gets easier.

1. Tarpey, P. et al. Nat. Genet. 41, 535–543 (2009).2. Stevenson, R., Schwartz, C. & Schroer, R. X-Linked

Mental Retardation (Oxford University Press, Oxford, 2000).

3. Chiurazzi, P., Schwartz, C.E., Gecz, J. & Neri, G. Eur. J. Hum. Genet. 16, 422–434 (2008).

4. Ropers, H.H. Curr. Opin. Genet. Dev. 16, 260–269 (2006).

5. Verkerk, A.J. et al. Cell 65, 905–914 (1991).6. Ropers, H.H. Curr. Opin. Genet. Dev. 18, 241–250

(2008).7. Levy, S. et al. PLoS Biol. 5, e254 (2007).8. Wheeler, D.A. et al. Nature 452, 872–876 (2008).9. Froyen, G. et al. Am. J. Hum. Genet. 82, 432–443

(2008).10. Lugtenberg, D. et al. Eur. J. Hum. Genet. 17, 444–453

(2009).

groups currently engaged in such large-scale sequencing surveys for discovering genes associated with human diseases. It is clear from the current work that a major chal-lenge for this era of resequencing studies is discerning causative variation, as these will be accompanied by many other changes that each look as if they might perturb gene function. In the absence of clear functional criteria, compelling evidence of association is required to make the case that alleles are causative. As it is unlikely that there will be any simple single solution to providing func-tional data for all the discovered variation, the process of assigning phenotypes to newly

sequences (promoters, intronic regions) of genes or in non-protein-coding genes such as miRNA sequences. They also suggest that copy number variants or small inversions could play a role and would be missed with their approach9,10. It is possible that some of the missense mutations already identified could be causative, but this is more difficult to demonstrate. The difficulty escalates when considering changes in promoters, introns and other noncoding sequences.

Lessons for medical sequencingThis study echoes old lessons from mende-lian genetics that should be heard by other

Of flaky tails and itchy skinDonata Vercelli

a new study defines the flaky tail mouse as a model for human atopic dermatitis caused by a null mutation in the gene encoding filaggrin, a key component of the epidermal barrier. Research in these mice will help explain how a disrupted barrier contributes to the pathogenesis of atopic dermatitis and to asthma arising in the context of atopic skin disease.

Donata Vercelli is at the Arizona Initiative for the Biology of Complex Diseases (ABCD) and Arizona Respiratory Center, University of Arizona, Tucson, Arizona, USA. e-mail: donata@arc.arizona.edu

Among the progeny of a cross between stocks of heterogeneous origin … main-tained at The Jackson Laboratory in 1958 were some mice showing ears smaller than normal… The condition was due to a new recessive mutation. Because of an additional manifestation of tail constric-tions accompanied by flaking tail skin, the mutation was called flaky tail (ft)1.

These words described a spontaneous muta-tion that was first reported in 1972 and that is now expected to provide new insights into the pathogenesis of atopic dermatitis (eczema), a complex skin disease affecting at least 15% of children2. On page 602 of this issue, Fallon et al.3 characterize the ft mutation as a 1-bp dele-tion in the gene encoding filaggrin that virtu-ally abrogates the synthesis of this protein. This mutation closely resembles the human loss-of-function FLG variants that are responsible for the keratinizing disorder ichthyosis vulgaris4 and that confer major genetic risk for atopic dermatitis and atopic dermatitis–associated asthma5,6. Notably, Fallon et al. found that

application of allergen to intact skin of ft/ft mice was sufficient to induce cutaneous inflammatory infiltrates and development of allergen-specific antibody and cytokine responses reminiscent of those seen in human atopic dermatitis3.

Filaggrin is a key structural protein that facilitates terminal differentiation of the epi-dermis and formation of the skin barrier7. The strong association between FLG muta-tions and atopic dermatitis5, first identified in 2006, marked a milestone in the genetics of complex allergic disorders. Although previous candidate gene association studies for atopic dermatitis had focused mostly on immuno-logical pathways, the finding that FLG variants strongly predispose to atopic dermatitis, and to asthma in the context of atopic dermatitis, shifted attention to the epithelium and to the role of a disrupted epidermal barrier in grant-ing access to allergens that trigger systemic and local lung inflammatory responses8. It is noteworthy that, although between 18% and 48% of all eczema collections carry FLG null alleles9, the combined allele frequency of FLG variants in the general European population is approximately 9% (Fig. 1)7. However, because these variants were not (and still are not) repre-sented on genotyping chips commonly used for genome-wide association studies, FLG muta-tions might have gone unnoticed if they had not been previously found to cause ichthyosis

vulgaris, which often coexists with atopic der-matitis and in few cases also with asthma4.

From mice to men and backThe impressive results of these human genetic studies set the stage for the generation of ani-mal models that could allow a mechanistic dissection of the role of filaggrin in skin bar-rier function in health and disease. Initially regarded as a model for filaggrin-deficient ichthyosis vulgaris10, the ft mouse promises to become a powerful model for atopic dermati-tis. Several crucial mechanistic questions about disease pathogenesis and the role of filaggrin may be answered by studying ft mice. What is the nature of the danger signal generated in the skin milieu that triggers the development of an immune response? What are the relative roles of Th2, Th17 and Th1 cells in disease patho-genesis? Which innate and adaptive immu-nological pathways mediate the activation of effector responses in the skin and the link with lung inflammation? And how does filaggrin deficiency facilitate growth of Staphylococcus aureus, which colonizes the skin of more than 90% of individuals with atopic dermatitis and contributes to phenotypic exacerbations7?

On the other hand, two characteristics of ft mice remain puzzling. First, percutaneously sensitized ft/ft mice failed to develop lung inflammation and airway hyper-responsive-ness after aerosol allergen challenge despite

©20

09 N

atu

re A

mer

ica,

Inc.

All

rig

hts

res

erve

d.

news and v iews

nature genetics | volume 41 | number 5 | may 2009 513

40% of carriers of FLG null alleles have no signs of atopic dermatitis7. The environmen-tal and genetic modifiers of this risk are cur-rently unclear, but some clues are emerging and may guide future mechanistic research in ft mice (Fig. 1). For instance, IL-4 and IL-13, the Th2 cytokines that induce allergic responses, have been shown to decrease fil-aggrin skin expression12, thus contributing to the skin barrier defect in individuals with atopic dermatitis. Variants in IL13 and IL4 are associated with atopic dermatitis13 and might interact with FLG mutations. Moreover, a recent study highlighted an intriguing gene–environment interaction involving FLG and neonatal exposure to pets. Exposure to cat, but not dog, at birth substantially increased the risk of atopic dermatitis in the first year of life in children with FLG loss-of-function variants, but not in children without those variants14. Interestingly, these effects seemed to be independent of early allergic sensitiza-tion to cat, which is consistent with the dem-onstration that the effects of FLG mutations were equally strong on allergic and nonal-lergic eczema15. Cat-associated microbial products might more easily penetrate a skin barrier compromised by abnormal filaggrin biosynthesis and provide signals that trig-ger skin inflammation. Moving ft mice to a Th2- or IgE-deficient background will define the role of allergic responses in murine atopic dermatitis, and exposing these mice to relevant stimuli will identify exogenous and endogenous pathways that influence filaggrin-dependent events. Ultimately, the ft model will likely shed light on gene–gene and gene–environment interactions that modulate disease pathogenesis by modify-ing filaggrin expression and function.

1. Lane, P.W. J. Hered. 63, 135–140 (1972).2. Leung, D.Y. & Bieber, T. Lancet 361, 151–160

(2003).3. Fallon, P.G. et al. Nat. Genet. 41, 602–608 (2009). 4. Smith, F.J. et al. Nat. Genet. 38, 337–342 (2006).5. Palmer, C.N. et al. Nat. Genet. 38, 441–446 (2006).6. Baurecht, H. et al. J. Allergy Clin. Immunol. 120,

1406–1412 (2007).7. O’Regan, G.M., Sandilands, A., McLean, W.H. &

Irvine, A.D. J. Allergy Clin. Immunol. 122, 689–693 (2008).

8. Hudson, T.J. Nat. Genet. 38, 399–400 (2006).9. Irvine, A.D. J. Invest. Dermatol. 127, 504–507

(2007).10. Presland, R.B. et al. J. Invest. Dermatol. 115, 1072–

1081 (2000).11. Henderson, J. et al. J. Allergy Clin. Immunol. 121,

872–879 (2008).12. Howell, M.D. et al. J. Allergy Clin. Immunol. 120,

150–155 (2007).13. Brown, S.J. & McLean, W.H. J. Invest. Dermatol. 129,

543–552 (2009).14. Bisgaard, H. et al. PLoS Med. 5, e131 (2008).15. Marenholz, I. et al. J. Allergy Clin. Immunol. 118,

866–871 (2006).

normal except for the shortened ears and loss of tail tips10. Human ichthyosis vulgaris and atopic dermatitis also improve with age, perhaps reflecting a decreased need for filag-grin that coincides with a decline in filaggrin expression with age10. In the current study3, ft/ft mice were epicutaneously sensitized and tested for airway responsiveness at adult age (6–8 weeks). The lack of lung responses, and even the recessive nature of the genetic defect, may therefore reflect treatment of the mice at an age at which the role of filaggrin in main-taining barrier integrity has become less criti-cal, and lung development and function are less readily affected. Experiments in younger mice will clarify these issues, potentially enhancing the significance of the ft model.

Genetic and environmental modifiersAlthough the association between FLG muta-tions and atopic dermatitis is strong, around

vigorous systemic allergen-induced inflam-mation and antibody production3. Moreover, an atopic dermatitis–related phenotype was detected only in mice homozygous for the ft mutation3. Both findings are at odds with human population data showing that heterozy-gosity for null FLG mutations is sufficient to predispose not only to atopic dermatitis but also to atopic dermatitis–associated asthma11. Although the choice of the C57BL/6 strain, which is less airway reactive than Balb/c, and the use of ovalbumin, an allergen of uncertain clinical significance, may have contributed to these experimental outcomes3, another more interesting possibility looms large. The ft mouse phenotype presents a strong devel-opmental component10. Homozygous ft mice appear normal at birth. The flaky tail pheno-type appears at about 3 days of age and peaks around day 6. Later, the phenotype gradually improves, until at day 15–21 the pups appear

Figure 1 Phenotypic heterogeneity in atopic dermatitis. Among carriers of FLG null mutations (9% of the European population; orange silhouette), a substantial proportion (40%) shows no disease phenotype (white silhouette). This heterogeneity likely reflects the existence of genetic and environmental modifiers, some of which are known or postulated. Mechanistic studies in ft mice might identify additional ones. Image of skin with atopic dermatitis provided by I. McLean (University of Dundee) and A. Irvine (Our Lady’s Children’s Hospital and Trinity College Dublin).

©20

09 N

atu

re A

mer

ica,

Inc.

All

rig

hts

res

erve

d.